1. Field of the Invention
The present invention relates to a nitride semiconductor laser chip and to a method for fabricating it. More specifically, the invention relates to a nitride semiconductor laser chip having nitride semiconductor layers laminated on a nitride semiconductor substrate having a particular planar orientation, and to a method for fabricating such a nitride semiconductor laser chip.
2. Description of Related Art
Nitride semiconductors are compounds of N (nitrogen), which is a group V element, with a group III element, such as Al (aluminum), Ga (gallium), and In (indium). For their band structures and chemical stability, nitride semiconductors have been receiving much attention as semiconductor materials for light-emitting devices and power devices, and have been tried in various applications. Especially active is the development of nitride semiconductor laser chips that emit light in the ultraviolet to visible region as light sources for optical information recording apparatuses, illumination apparatuses, display apparatuses, sensors, etc.
In a nitride semiconductor laser chip, it is common to use a nitride semiconductor substrate, that is, a substrate of the same type of material as the nitride semiconductor layers to be laminated on its surface. This helps enhance the quality of the laminated nitride semiconductor layers and thereby enhance the characteristics of the semiconductor laser chip. Typically used as such a substrate is, for its ease of fabrication, a crystal having a wurtzite structure and having the (0001) plane as its principal plane. When a crystal of nitride semiconductor layers is formed on this substrate, it grows, likewise, with the (0001) plane as its principal plane.
With such a semiconductor laser chip having nitride semiconductors laminated with the (0001) plane as the principal plane, that is, having them laminated in the [0001] direction (in the C-axis direction), there is concern for a drop in the electron hole recombination probability ascribable to the Stark effect under the influence of an internal electric field in a quantum well active layer. As a chip structure that alleviates this disadvantage, there have also been studied nitride semiconductor laser chips having a laminate structure formed in the direction perpendicular to the C-axis (see JP-A-H8-213692 and JP-A-H10-51029).
With a nitride semiconductor laser chip laminated in the direction perpendicular to the C-axis in this way, the reduced influence of the Stark effect and the increased crystal asymmetry in the quantum well plane are expected to lead to an increased gain, and moreover the suppression of the penetrating dislocation, which tends to develop in the C-axis direction, developing in the lamination direction is expected to lead to enhanced crystallinity, and hence a reduced threshold current density. All this predicts high-performance chip characteristics with excellent reliability.
In an expression representing a plane or orientation of a crystal, it is convention in crystallography to signify a negative index by putting a horizontal line over its absolute value; in the present specification, however, since that notation cannot be adopted, a negative index is instead signified by putting a minus sign “−” before its absolute value.
Disadvantageously, however, even a nitride semiconductor laser chip laminated on a nitride semiconductor substrate having the (1-100) plane as the principal plane as described above does not offer satisfactory characteristics: when such a chip is subjected to CW (continuous wave) lasing (continuous lasing) up to a high output, a certain proportion of its samples (individual chips actually fabricated) break down before reaching a satisfactory light output.
Moreover, with this conventional nitride semiconductor laser chip, the proportion of its samples that break down increases with the length of time for which they are driven. Depending on the conditions under which they are driven, it may even occur that most samples of the nitride semiconductor laser chip fabricated offer unsatisfactory reliability. This indicates that a nitride semiconductor laser chip laminated on a nitride semiconductor substrate having the (1-100) plane as the principal plane suffers from, as inherent in its characteristics, inconveniences that cannot be overcome with conventional knowledge, specifically the disadvantage of extremely low yields of acceptable samples and the risk of sudden breakdown in actual use for a long time.
In search of the cause, therefore, the inventors of the present invention have done intensive studies, and as a result have found out the following facts: at resonator (cavity) facets, in an active layer, a step (unflushness) develops parallel to nitride semiconductor layers, resulting in poor flatness; furthermore, the step causes damage to the crystal nearby, and also causes unsatisfactory adhesion of coating film around and hence poor protection of the facet, resulting in poor resistance to damage at laser resonator facets.